The use of electrical measurements in prior art downhole applications, such as logging while drilling (LWD), measurement while drilling (MWD), and wireline logging applications is well known. Such techniques may be utilized to determine a subterranean formation resistivity, which, along with formation porosity measurements, is often used to indicate the presence of hydrocarbons in the formation. For example, it is known in the art that porous formations having a high electrical resistivity often contain hydrocarbons, such as crude oil, while porous formations having a low electrical resistivity are often water saturated. It will be appreciated that the terms resistivity and conductivity are often used interchangeably in the art. Those of ordinary skill in the art will readily recognize that these quantities are reciprocals and that one may be converted to the other via simple mathematical calculations. Mention of one or the other herein is for convenience of description, and is not intended in a limiting sense.
Directional resistivity measurements are also commonly utilized to provide information about remote geological features (e.g., remote beds, bed boundaries, and/or fluid contacts) not intercepted by the measurement tool. Such information includes, for example, the distance from and direction to the remote feature. In geosteering applications, directional resistivity measurements may be utilized in making steering decisions for subsequent drilling of the borehole. For example, an essentially horizontal section of a borehole may be routed through a thin oil bearing layer. Due to the dips and faults that may occur in the various layers that make up the strata, the distance between a bed boundary and the drill bit may be subject to change during drilling. Real-time distance and direction measurements may enable the operator to adjust the drilling course so as to maintain the bit at some predetermined distance from the boundary layer. Directional resistivity measurements also enable valuable geological information to be estimated, for example, including the dip and strike angles of the boundary as well as the vertical and horizontal conductivities of the formation.
Methods are known in the art for making LWD directional resistivity measurements. Directional resistivity measurements commonly involve transmitting and/or receiving transverse (x-mode or y-mode) or mixed mode (e.g., mixed x- and z-mode) electromagnetic waves. Various tool configurations are known in the art for making such measurements. For example, U.S. Pat. No. 6,181,138 to Hagiwara teaches a method that employs an axial (z-mode) transmitting antenna and three co-located, circumferentially offset tilted receiving antennae. U.S. Pat. Nos. 6,969,994 to Minerbo et al., 7,202,670 to Omeragic et al., and 7,382,135 to Li et al teach a method that employs an axial transmitting antenna and two axially spaced tilted receiving antennae. The receiving antennae are further circumferentially offset from one another by an angle of 180 degrees. U.S. Pat. Nos. 6,476,609, 6,911,824, 7,019,528, 7,138,803, and 7,265,552 to Bittar teach a method that employs an axial transmitting antenna and two axially spaced tilted receiving antennae in which the tilted antennae are tilted in the same direction. U.S. Pat. Nos. 7,057,392 and 7,414,407 to Wang et al teach a method that employs an axial transmitting antenna and two longitudinally spaced transverse receiving antennae.
One difficulty in making LWD resistivity measurements (both conventional and directional measurements) is constructing transmitting and receiving antennae that are capable of withstanding the demanding downhole conditions. As is known to those of ordinary skill in the art, LWD tools are routinely subject to severe mechanical impacts with the borehole wall and with cuttings in the borehole fluid. These impacts would quickly destroy the sensitive antenna components if they were left unprotected. Conventional LWD resistivity tools commonly employ shields to physically protect the antennae. Suitable antenna shields must provide sufficient mechanical protection without distorting and/or over-attenuating the transmitted and/or received electromagnetic waves. In practice virtually all antenna shields that provide suitable protection also attenuate or selectively attenuate the electromagnetic waves due to the physical barrier that they provide. There is a difficult practical tradeoff in configuring an antenna shield that provides sufficient mechanical protection and has a low, non-selective attenuation. In general, highly protective shields tend also to be highly attenuating.
Conventional LWD resistivity tools commonly employ shields having slots (or apertures) formed therein. For example, U.S. Pat. No. 5,530,358 to Wisler et al discloses an LWD tool having a plurality of circumferentially spaced, axial slots formed in the outer surface of the tool housing. The use of a protective sleeve having axial slots is also known. Such antenna shields are known to provide adequate mechanical protection with sufficiently low attenuation of axial (z-mode) electromagnetic waves. Axially slotted shields are therefore commonly used in non-directional (z-mode) resistivity tools.
While certain axially slotted shields are known to exhibit sufficiently low attenuation to axial electromagnetic waves, these shields are also known to highly attenuate and distort transverse (x- and y-mode) electromagnetic waves. As such, the conventional wisdom in the art is that axially slotted shields are unsuitable for use with directional resistivity antennae (antennae that are configured to transmit and/or receive transverse mode or mixed mode electromagnetic waves). Directional resistivity tools therefore commonly employ sloped, curved, and/or circumferential slots. For example, U.S. Pat. No. 6,297,639 to Clark et at discloses a directional resistivity tool having a plurality of sloped (non-axial) and/or curved slots formed in an outer surface of the tool body. U.S. Pat. No. 6,566,881 to Omeragie et al discloses a tool having a plurality of axially spaced, circumferential slots formed in the tool body. U.S. Pat. No. 7,057,392 to Wang et al discloses a directional resistivity tool having a plurality of transversal slots formed in an outer surface of the tool body to protect transversal antennas.
When there is a desire to substantially collocate a non-directional (axial) antenna with a directional antenna to perform a multi-component measurement, multidirectional slots are often employed. These slots can compromise the structural integrity of the tool. In addition, fabrication of drill collars having multiple sloped, curved, and/or circumferential slots typically requires complex and expensive machining operations. Therefore, there is a need in the art for an improved antenna shield to be used in a directional resistivity tool.